The present invention relates generally to electrical switches and more particularly to piezoelectric switches.
Piezoelectric switches, also commonly referred to simply as piezo switches, are well known devices. Piezoelectric switches typically comprise a piezo crystal transducer, or piezo element, which is electrically connected to an electronic circuit board by a conductor, such as a pair of wires. The piezo crystal transducer, electronic circuit board and conductor are typically held within a plastic housing. The piezo crystal transducer is typically mounted within the plastic housing so that an input of physical contact onto the housing, in turn, produces an input of physical contact onto the piezo crystal transducer.
In use, the input of physical contact, such as by tactile pressure, onto the housing of the piezoelectric switch, in turn, produces an input of physical contact onto the piezo crystal transducer. The input of physical contact onto the piezo crystal transducer causes the piezo crystal transducer to produce an electrical output that passes onto the electronic circuit board which, in turn, effects the state of connection of the piezoelectric switch.
In U.S. Pat. No. 4,430,595 to H. Nakasone et al, there is disclosed a piezo-electric push button switch. In the piezo-electric push button switch, a flexible printed circuit board is secured to terminals which are embedded in the housing, and the piezo-electric plate is supported by the flexible printed circuit board in such a manner that it is floated from the housing, whereby external impact or vibration is decreased, and in which an integrated circuit consisting of a resistor and a capacitor is incorporated in the switch body, to eliminate harmonics.
Piezoelectric switches are more desirable than conventional mechanical switches because piezoelectric switches have no moving parts. To the contrary, conventional mechanical switches have mechanical contacts which depend upon significant freedom of motion of at least one component for enabling the switch to function properly. Accordingly, it should be known that utilizing a piezoelectric switch, which has no moving parts, introduces numerous advantages.
As a first advantage, because a piezoelectric switch has no moving parts, the switch is less likely to become damaged from use, thereby making the switch more reliable over time.
As a second advantage, because a piezoelectric switch has no moving parts, the switch can be filled with a potting compound, such as a silicon gel, in order to make the switch hermetically sealed against environmental changes which can either actuate the switch, such as by a change in temperature, or which can corrode the switch, such as by the introduction of moisture into the switch.
For example, in U.S. Pat. No. 4,618,797 to D. J. Cline, there is disclosed an environmentally sealed sensing assembly for an electrical switch for use in adverse conditions such as a warm and humid location. The sensing assembly comprises a piezoelectric element carried by a relatively thin, flexible member supported at its edges within a recess formed in an enclosure. The enclosure includes a cavity in communication with the recess through a flow passage. A resiliently deformable encapsulating or potting material fills the recess, cavity, flow passage and any voids adjacent the piezoelectric element and the flexible member. Flexure of the flexible member through manual pressure or the like, to deform the piezoelectric element, is permitted by resilient deformation or "flow" of the potting compound. The potting material thus provides sealing while yet allowing mechanical movement of the flexible member and associated piezoelectric element. The resulting electrical signal caused by deformation of the piezoelectric element can be conditioned and used for switching electrical devices.
Environmentally sealed piezoelectric switches of the type described in Cline have been found to experience some notable drawbacks.
As a first drawback, it has been found that soldering a pair of conductive wires onto the piezoelectric element can be a difficult task because of the relatively small surface area of the piezoelectric crystal. Specifically, piezoelectric elements used in piezoelectric switches typically comprise a top layer which is formed entirely of piezo crystal and a slightly larger bottom layer which is formed entirely of brass. Accordingly, because the bottom layer of brass is often larger in size than the top layer of piezo crystal, a conductive wire must be soldered onto the top surface of each layer of the piezoelectric element. As such, it has often been found that this means of connection can cause considerable interference of the wires against the housing of the switch, which is undesirable.
As a second drawback, it has been found that the user is often unable to determine the state of the piezoelectric switch. Specifically, due to the lack of moving parts in piezoelectric switches, the user is often unaware of whether the state of the switch has been changed upon the input of pressure onto the housing.
As a result, in U.S. Pat. No. 5,636,729 to R. Wiciel, there is disclosed a piezo-electric switch having a lighted push-button. The piezo switch has a case for mounting the push-button, a piezo element with a hole in it, and a hollow spacer for separating the push-button from the piezo element; the holes in the spacer and piezo element are aligned with each other. Within the case is mounted a light source, where the light source may be positioned within the hole in the spacer, or in the hole in the piezo element, but in either configuration allowing light to shine through the holes to illuminate the push-button.
Although well-known and widely used in commerce, the piezoelectric switch disclosed in Wiciel includes additional components, such as a light source, which can significantly increase the overall cost of the switch.